Transect Sampling Methods for a Minority Population Genetic Epidemiology Study

1
Transect Sampling Methods for a Minority
Population Genetic Epidemiology Study

• Nathan K. Risk, M.A.
• Krista L. Russell, B.A.
• Rumi Kato Price, Ph.D., M.P.E.

This study is supported by the Missouri Alcohol
Research Center (MARC). (P50AA1198, Center
Director Andrew C. Heath, D. Phil)
2
Abstract
The Saint Louis Asian American Pilot Study is
conducted under the auspices of the Missouri
Alcohol Research Center (MARC). It proposes to
conduct a cross sectional assessment in two Asian
sub-populations, the Japanese and the Vietnamese
residing in the Saint Louis Area. Current
population-based sampling methods that require
sampling frames are unlikely to be suitable for
efficiently sampling members of a small
population. The population distribution in the
Saint Louis area is not large enough to make the
census-based sampling method reasonable. Although
there are a number of other sampling methods that
do not require sampling frames, the transect
sampling derived originally from wildlife biology
is a particularly flexible method. This
presentation will show 1) the need for
ascertaining the Asian sub-population in the
United States 2) the basics of the transect
sampling method in wild-life biology 3) the
adaptation of the transect sampling method for
the Saint Louis Asian population and 4)
simulated data that examines the robustness of
transect sampling under various assumptions.
3
Introduction (1)

In Table 1, the rates of current drinking are
reported separately for people of Asian decent
who self identify as white versus people of Asian
descent who self-identify as Asians using
NLAES1. The results show that the rates of
drinking are higher among Asians who self
identify as white2. In Table 2, the rates of
current drinking are reported separately for
unmixed and mixed Asian adolescents using Add
Health.3 The results consistently show that the
rates of current drinking are higher among Asians
of mixed heritage2. It is an aim of this pilot
study to examine the relative strength of
acculturation measures such as mixed heritage on
substance use and problem use in these two
community samples. The deficiency of the ALDH2
isozyme is known to cause a high sensitivity to
alcohol among Asians4. This sensitivity is known
as the flushing syndrome. There is also some
evidence for a correlation of the wild type
CYP2A61 with tobacco dependence5. Mutations of
CYP2A6 have been reported at a higher frequency
among Asians6,7. Genotypic frequencies are
given in Table 3. It is an aim of the pilot study
to assess the feasibility of genotype collection
for these two alleles for the Japanese and
Vietnamese populations in Saint Louis, since
genotypic distributions among Asians in the U.S.
are expected to be substantively different from
those in their native lands given the high rate
of inter-racial reproduction currently on-going
in the U.S.
4
Table1. Drinking by Racial Identity () (Asian
n922)
Source NLAES, 1992, Price, et. al., 2002. ,
rate significantly higher among those identified
themselves as whites , unreliable estimates
due to small numbers of Asians who identified
themselves as whites.
5
Table 2. Drinking among Adolescent Asians by
Multiple Racial Identity1
(Asian n 4,012)
Racial identity Japanese Filipino Chinese
Korean Vietnamese Unmixed Asian 24.5 24.3
9.7 17.1 12.9 Mixed Asian 39.2 40.6 41.7
33.7 49.4 Source Add Health S-95, Price et
al, 2002. 1. Mixed Asian are those who reported
at least one more race. Multiple choices for
Asian ethnicity allowed. Weighted to be
generalizable to the U.S. Population of
adolescents in grade 7 through 12 in 1994-6.
Standard errors adjusted using SUDAAN. ,
significantly larger than unmixed Asians.
6
Table 3. Genotype Distributions of ALDH2 and
CYP2A6
ALDH2
CYP2A6
Genotypes ()
Allelic () Frequency
1
2
n
n
1/1
2/1
2/2
61
62(
1)
63(
3)
3
4
4
4
Caucasian
100
0
0
85
15
0
5
5
?
98.6
1.4
Japanese
56.4
39.4
4.2
52
20
28
Filipino
87.3
12.7
0
Not Available
Korean
71.6
26.6
1.8
Not Available
6
6
6
Chinese
59.0
35.9
5.1
83
11
6
Vietnamese
43.0
57.0
Not Available

Source Harada, 1991 Goedde et al., 1985 1.
Wildtype 2. Inactive 3. European 4. Finnish based
on the PCR amplifisters confined with diagnostic
restriction digestion. 5. Finnish, based on a
two-step PCR method 6. Taiwanese
7
Introduction (2)

In any study of a minority population,
ascertainment of a representative sample is a
challenge. It is possible to ascertain a sample
randomly by selecting blindly from the total
population and accepting only respondents from
the minority in question, but such an approach is
prohibitively expensive. It is also tempting to
enter the minority community at a few points and
select respondents from a few well known centers
of the community. Such an approach may well
succeed in meeting the ascertainment goal, but
the sample may not reflect the minority community
under study as a whole. In wild-life biology,
transect sampling is used to ascertain a
representative sample of the population of a
given species by traveling along randomly chosen
paths or transects. Observations of the species
are noted and the distance to the observation
from the transect is recorded (Figure 1). Since
members of a species further from the transect
are less likely to be spotted, the density of the
species is estimated by fitting the percentage of
observations made to distance. Several parametric
methods for this fitting exist8. (Figure 2)
8
Figure 1. Transect Sampling Along Paths
Observation
Transect
Distance

Blow-up detail of an observation and a transect.
9
Figure 2. Methods of Density Estimation in
Transect Sampling
Density of Detections
Density of Detections
Distance
Distance
Exponential
Half - Normal
Source Thompson, 1992.
10
Method (1)
This pilot study adapts a transect sampling
method for the purpose of ascertaining the
Japanese and Vietnamese samples residing in Saint
Louis (Table 5 on sister poster.) The Japanese
sub-population is centered in two bands,
Brentwood to Chesterfield and University City to
Olivette (Figure 3). The Vietnamese population
is also centered in two bands, Olivette to
Maryland Heights and South City to South County
(Figure 4). The entry points into this
population will be by community organizations and
retail services that serve the minority
community. Instead of the path method commonly
used in transect sampling, each entry point into
the community will recruit by advertisement. For
the ascertained sample to successfully represent
the community, the entry points must be scattered
thoroughly throughout the community. By selecting
a large variety of community organizations and
retail services (Figure 5), the pilot study hopes
to ascertain a representative sample from the
respective populations in a cost-efficient
manner.
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Figure 3. Distribution of the Japanese
Population in Saint Louis
Brentwood-Chesterfield University City-Olivette
12
Figure 4. Distribution of the Vietnamese
Population in Saint Louis
13
Figure 5. Map of Community Organizations and
Service Providers1
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1. The list is recompiled periodically.
14
Method (2)
In order to test the effectiveness of transect
sampling, several simulations were performed.The
basic simulation was a four-strata simulation.
The strata were geographically assigned to the
four corners of the sample space (Figure 6).
This mimics the actual geographic lay-out of the
Japanese residing in Saint Louis County. Twenty
entry points were placed among the strata. Each
entry point was given a randomly assigned
popularity score. Observations were randomly
assigned to a location inside of their strata and
each observation was randomly assigned a binary
score (such as mixed- heritage or high-school
education) for Traits A and B based on the strata
they were in. For each entry point, a person
(observation) would randomly visit that entry
point based on the popularity of the entry point
and the distance of the person from the entry
point. A person visiting any entry point was
considered to be sampled. For Simulation 1, all
observations behaved similarly. For Simulations
2-5, observations from strata 1 were less likely
to visit distant entry points and hence less
likely to be sampled while observations from
strata 2 were more likely to visit distant entry
points and hence more likely to be sampled.
15
Figure 6. Four Strata Simulation with 20 Entry
Points
Strata 1
Strata 3
Strata 2
Strata 4
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Results

• For Simulation 1 (Table 4), when all members of
  the population respond similarly to the distance
  to entry points, each strata is sampled at a
  similar rate (18.6, 18.8, 18.6, 18.9) and
  therefore the sample proportion of Traits A and B
  is similar to the population proportion of Traits
  A and B (47.1 versus 47.3 and 32.1 versus 32.1
  respectively)
• For Simulations 2-5, as strata 1 becomes sampled
  at an increasingly lower rate and strata 2
  becomes sampled at an increasingly higher rate,
  the sample proportions of Traits A and B vary to
  an increasing extent from the population
  proportion of Traits A and B.
• The estimator of pi, pi is generally adequate,
  but tends to be lower than pi, since it is biased
  low by definition. An increased number of entry
  points and an even sampling across entry points
  will reduced this bias.
• Transect sampling is a reasonable method to
  obtain a representative sample if enough entry
  points are provided for each strata of the
  population. It is also feasible to measure the
  effectiveness of this sampling strategy for
  equality of sampling across strata.

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Table 4. Simulation Results
Traits A and B give the sampled proportion of
Traits A and B in each simulation. pi is the
sampling proportion of strata i, (i.e. the
proportion of observations that visited any of
the 20 entry points.)
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Literature Cited
1. National Institute on Alcohol and Abuse and
Alcoholism. National Longitudinal Epidemioligic
Alcohol Survey Wave 1 Questionnaire. Rockville
(MD) NIAAA 1991. 2. Price, RK, et al. Substance
Use and Abuse by Asian Americans and Pacific
Islanders Preliminary Results from Four National
Epidemioligic Studies. Public Health Reports
2002 39-49 3. National Longitudinal Study of
Adolescent Health. Research Design. Chapel Hill
(NC) Carolina Population Center, Univ of North
Carolina 1998. 4. Crabb DW, et al. Genetic
factors that reduce risk for developing
alcoholism in animals and humans. In Begleiter H,
Kissin B (eds.), The genetics of alcoholism. New
York (NY) Oxford University Press, 1995
202-220 5. Wellington C. Genes and tobacco
dependence (a review). Clin Genet 1998
54266-267. 6. Yokoi T, Kamataki T. Genetic
polymorphism of drug metabolizing enzymes New
mutations in CYP2D6 and CYP2A6 genes in Japanese.
Pharmaceut Res 1998 15517-524. 7. Oscarson M,
et. al .Characterisation and PCR-based detection
of a CYP2A6 gene deletion found at a high
frequency in a Chinese population. FEBS Letters
1999 448105-110. 8. Thompson, S.Sampling. New
York (NY) John Wiley and Sons, Inc., 1992